In early types of electron microscope, the sample to be imaged was contained in a sample chamber in which a vacuum was maintained during the imaging process. However, there are occasions on which it is necessary or desirable to image specimens in a gaseous environment in a scanning electron microscope. For example the gaseous environment would inhibit the evaporation of moisture from a biological specimen, and can dissipate surface charges from non-conductive specimens, which charges would otherwise accumulate to the detriment of image resolution.
The use of a gaseous environment amplify a secondary electron signal obtained during imaging of the specimen is described in U.S. Pat. No. 4,785,182 (Mancuso et al.) and European Patent No. EP 330310 (Electroscan Corp). In both arrangements, secondary electrons were released from a specimen as a result of interactions with a scanning electron beam, and aerated through the gaseous environment in the sample chamber. The resulting collisions of the secondary electron with the gas molecules of the environment ionised the molecules, and thus released further electrons which were also accelerated so as to give rise to further collisions and the release of yet further electrons. Thus the secondary electrons emanating from the sample gave rise to an avalanche of electrons released by the ionisation of the gas molecules. Thus the gaseous environment, in effect, provides amplification of the secondary electrons.
However, the received secondary electron signal may include interference as the result of the interaction of backscattered electrons with the gaseous medium, as the backscattered electrons can also cause the gas medium to release an avalanche of electrons.
As the pressure of gas in the chamber is increased, the mean free path of the secondary electrons directly released from the sample decreases to the extent that many secondary electrons collide with molecules at velocities sufficient to cause the necessary ionisation for gas amplification to recur. Backscattered electrons however, have a larger mean free path than secondary electrons and will therefore make a greater contribution to the output signal at high gas pressures. U.S. Pat. No. 5,412,211 (Electronscan Corporation) and European Patent No. EP 0753200 (Phillips Electronics North America Corporation) describe the use of a biased pressure limiting aperture to reduce the backscattered contribution to the detected signal. However, this approach relies on rile backscattered electrons not travelling tough the same part of the sample chamber as the secondary electrons to be detected, an assumption which does not always bold.